Broadband optical spectrum generating apparatus and pulsed light generating apparatus

ABSTRACT

A broadband optical spectrum generating apparatus  20  includes a ultrashort pulsed fiber laser  22  that generates pulsed light having a pulse width in the unit of picosecond to femtosecond, and broadband optical spectrum-generating optical fibers  24  that are connected with the ultrashort pulse fiber laser  22  via a lens  26  and have characteristics of normal dispersion, that is, a nonlinear coefficient of not less than 10 [W −1 km −1 ] and a magnitude of wavelength dispersion of not greater than a value 0 [ps/km/nm] with regard to a wavelength of the pulsed light. The resulting output is super continuum that is widely broadened to a wavelength band of 1400 nm to 1750 nm and is chirped to a linear characteristic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a broadband optical spectrum generatingapparatus and a pulsed light generating apparatus. More specifically theinvention pertains to a broadband optical spectrum generating apparatusthat generates a broadband optical spectrum as well as to a pulsed lightgenerating apparatus that generates pulsed light having a desiredwavelength in a predetermined wavelength band.

2. Description of the Prior Art

Widely broadened optical spectra called super continuum have recentlydrawn attention in the field of optical communication and opticalmeasurement. A diversity of broadband optical spectrum generatingapparatuses and techniques have been proposed to utilize optical fibershaving nonlinear effects and thereby generate the super continuum (forexample, JAPANESE PATENT LAID-OPEN GAZETTE No. 8-234249, No. 10-90737,No. 11-160744, and No. 11-174503).

Optical fibers of several hundred meters or even several kilometers are,however, required in such prior art broadband optical spectrumgenerating apparatuses and techniques, and are thus unpractical.Multiple amplification stages are required for excitation light, whichis used for generation of super continuum. This results in undesirablycomplicated construction and large size of the apparatus.

The object of the present invention is thus to provide a broadbandoptical spectrum generating apparatus that generates an easily handled,widely broadened optical spectrum. The object of the present inventionis also to attain size reduction and simplified construction of thebroadband optical spectrum generating apparatus. The broadband opticalspectrum generating apparatus of the invention aims at generating pulsedlight of an arbitrary wavelength out of a wide wavelength band.

SUMMARY OF THE INVENTION

In order to achieve at least a part of the aforementioned objects, abroadband optical spectrum generating apparatus and a pulsed lightgenerating apparatus of the present invention are structured as follows.

A broadband optical spectrum generating apparatus of the presentinvention is a broadband optical spectrum generating apparatus thatgenerates a broadband optical spectrum, the apparatus including:

-   -   a pulsed light source that outputs pulsed light having a pulsed        width in a unit of picosecond to femtosecond; and    -   spectrum generating optical fibers that have characteristics of        normal dispersion, that is, a nonlinear coefficient of not less        than 10 (W⁻¹km⁻¹) and a magnitude of wavelength dispersion of        not greater than a value 0 (ps/km/nm) with regard to a        wavelength of the pulsed light generated by the pulsed light        source, are adjusted to have a length in a range of 1 (m) to 10        (m), and convert the pulsed light input from the pulsed light        source into a broadband optical spectrum.

The broadband optical spectrum generating apparatus of the presentinvention uses the spectrum generating optical fibers that have thecharacteristics of normal dispersion, that is, the nonlinear coefficientof not less than 10 (W⁻¹km⁻¹) and the magnitude of wavelength dispersionof not greater than the value 0 (ps/km/nm), and that are adjusted tohave the length in the range of 1 (m) to 10 (m). This ensures generationof the broadband optical spectrum and makes a linear relationshipbetween the wavelength of the optical spectrum and the time ofappearance of the optical spectrum. Namely this enables generation ofthe broadband optical spectrum that is chirped to a linearcharacteristic. This arrangement facilitates the subsequent processingincluding a series of processing in the course of extracting an opticalspectrum of an arbitrary wavelength. The apparatus of the invention usesa single pulsed light source. This attains size reduction and simplifiedconstruction of the apparatus.

In the broadband optical spectrum generating apparatus of the invention,it is preferable that the pulsed light output from the pulsed lightsource has a central wavelength of about 1556 (nm), a pulse width in arange of 10 through 1000 femtoseconds, and a peak intensity of not lessthan 1 (kW). It is also preferable that the spectrum generating opticalfibers are characterized by a magnitude of wavelength dispersion around−10 (ps/km/nm) and a nonlinear coefficient around 15 (W⁻¹km⁻¹). Thepreferable spectrum generating optical fibers are of a polarizationmaintaining type and have a length of 5 (m). The optical spectrum tendsto be more widely broadened with the narrower pulse width, the greaterpeak intensity, the higher nonlinearity, and the smaller magnitude ofwavelength dispersion. When the pulse width and the magnitude ofwavelength dispersion are fixed, the optical spectrum is determined bythe product of the peak intensity and the nonlinear coefficient.

In the broadband optical spectrum generating apparatus of the invention,there may be provided with pulse compressing optical fibers that havecharacteristics of abnormal dispersion with regard to the wavelength ofthe pulsed light generated by the pulsed light source, and compress atime width of the pulsed light output from the pulsed light source tooutput the time width-compressed pulsed light to the spectrum generatingoptical fibers. This arrangement compresses the time width of the pulsedlight input into the spectrum generating optical fibers. Thiseffectively shortens the time of appearance of the optical spectrum in awhole wavelength domain while maintaining the linearity of the opticalspectrum. Here, it is preferable that the pulse compressing opticalfibers are characterized by a nonlinear coefficient in a range of 2 to10 (W⁻¹km⁻¹) and by a magnitude of wavelength dispersion of not lessthan a value 4 (ps/km/nm) and have a length in a range of 10 to 50 (cm).The length of the pulse compressing optical fibers tends to be shortenedwith the higher peak intensity of the input pulsed light and to beelongated with the wider pulse width.

A pulsed light generating apparatus of the invention is a pulsed lightgenerating apparatus that generates pulsed light of a desired wavelengthout of a predetermined wavelength band, the apparatus including:

-   -   a pulsed light source that outputs pulsed light having a pulsed        width in a unit of picosecond to femtosecond;    -   spectrum generating optical fibers that have characteristics of        normal dispersion, that is, a nonlinear coefficient of not less        than 10 (W⁻¹km⁻¹) and a magnitude of wavelength dispersion of        not greater than a value 0 (ps/km/nm) with regard to a        wavelength of the pulsed light generated by the pulsed light        source, are adjusted to have a length in a range of 1 (m) to 10        (m), and convert the pulsed light input from the pulsed light        source into a broadband optical spectrum;    -   a wavelength tunable filter that allows transmission of an        optical spectrum of an arbitrary wavelength out of the broadband        optical spectrum output from the spectrum generating optical        fibers; and    -   a wavelength selector that selects a wavelength of the optical        spectrum transmitting through the wavelength tunable filter, in        response to an electric signal.

In the pulsed light generating apparatus of the present invention, thepulsed light source and the spectrum generating optical fibers functionto generate the broadband optical spectrum. The wavelength selectorselects the wavelength of the optical spectrum. The wavelength tunablefilter allows transmission of the optical spectrum of the selectedwavelength. The optical spectrum of an arbitrary wavelength isaccordingly extracted. Namely this arrangement ensures generation of theoptical spectrum or the pulsed light of a desired wavelength out of abroad wavelength band.

In the pulsed light generating apparatus of the invention structured inthis way, it is preferable that the wavelength tunable filter is anacousto-optic wavelength tunable filter and the pulsed light sourceoutputs pulsed light having a central wavelength of about 1556 (nm), apulse width in a range of 10 to 1000 femtoseconds, and a peak intensityof not less than 1 (kW). It is also preferable that the spectrumgenerating optical fibers are characterized by a nonlinear coefficientaround 15 (W⁻¹km⁻¹) and a magnitude of wavelength dispersion around −10(ps/km/nm) and have a length of approximately 5 (m).

Moreover, in the pulsed light generating apparatus of the invention,there may be provided with pulse compressing optical fibers that havecharacteristics of abnormal dispersion with regard to the wavelength ofthe pulsed light generated by the pulsed light source, and compress atime width of the pulsed light output from the pulsed light source tooutput the time width-compressed pulsed light to the spectrum generatingoptical fibers. This arrangement compresses the time width of the pulsedlight input into the spectrum generating optical fibers. Thiseffectively shortens the time of appearance of the optical spectrum in awhole wavelength domain while maintaining the linearity of the opticalspectrum. Here, it is preferable that the pulse compressing opticalfibers are characterized by a nonlinear coefficient in a range of 2 to10 (W⁻¹km⁻¹) and by a magnitude of wavelength dispersion of not lessthan a value 4 (ps/km/nm) and have a length in a range of 10 to 50 (cm).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the construction of a broadband opticalspectrum generating apparatus 20 in one embodiment of the presentinvention;

FIG. 2 is a graph showing an observed output spectrum from broadbandoptical spectrum-generating optical fibers 24 having a fiber length of 1(m);

FIG. 3 is a graph showing an observed output spectrum from broadbandoptical spectrum-generating optical fibers 24 having a fiber length of 3(m);

FIG. 4 is a graph showing an observed output spectrum from broadbandoptical spectrum-generating optical fibers 24 having a fiber length of 5(m);

FIG. 5 is a graph showing variations in wavelength of the observedoutput spectra from the broadband optical spectrum-generating opticalfibers 24 plotted against the output of a ultrashort pulse fiber laser22;

FIG. 6 is a spectrogram showing the relationship between the wavelengthof the observed output spectra from the broadband opticalspectrum-generating optical fibers 24 having the fiber length of 1 (m)and the time of appearance;

FIG. 7 is a spectrogram showing the relationship between the wavelengthof the observed output spectra from the broadband opticalspectrum-generating optical fibers 24 having the fiber length of 5 (m)and the time of appearance;

FIG. 8 is a spectrogram showing the relationship between the wavelengthof the observed output spectra from broadband opticalspectrum-generating optical fibers of a comparative example and the timeof appearance;

FIG. 9 schematically illustrates the construction of a broadband opticalspectrum generating apparatus 20B in a second embodiment;

FIG. 10 is a graph showing the width of the output pulse plotted againstthe length of pulse compressing optical fibers 28 in one experiment;

FIG. 11 is a graph showing an observed output spectrum from pulsecompressing optical fibers 28 having a fiber length of 16 (cm);

FIG. 12 schematically illustrates the construction of a broadbandoptical spectrum generating apparatus 30 in one embodiment of thepresent invention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes preferred embodiments of the present invention.FIG. 1 schematically illustrates the construction of a broadband opticalspectrum generating apparatus 20 in one embodiment of the presentinvention. The broadband optical spectrum generating apparatus 20 of theembodiment includes a ultrashort pulse fiber laser 22 that generatespulsed light having a pulse width in the unit of picosecond tofemtosecond, and broadband optical spectrum-generating optical fibers 24that are connected with the ultrashort pulse fiber laser 22 via a lens26 to convert the pulsed width output from the ultrashort pulse fiberlaser 22 into a broadband optical spectrum.

The ultrashort pulse fiber laser 22 used in the embodiment stablyoutputs ultrashort pulsed light having a pulse width of 110 femtoseconds(fs), a central wavelength of 1556 (nm), a frequency of 50 (MHz), and amean output of 60 (mW). The pulsed light output from the ultrashortpulse fiber laser 22 preferably has the narrower pulse width and thehigher peak intensity. The high peak intensity of the output pulsedlight ensures the favorable results even under the condition of the widepulse width, based on the relationship between the pulse width and thepeak intensity of the output pulsed light. The preferable range of thepulse width is 10 to 1000 femtoseconds, and the preferable range of thepeak intensity is not less than 1 (kW).

Optical fibers of highly nonlinear normal dispersion shifted type arepreferably applied for the broadband optical spectrum-generating opticalfibers 24. The preferable optical fibers are characterized by anonlinear coefficient of not greater than 10 (W⁻¹km⁻¹) and a magnitudeof wavelength dispersion of not greater than a value 0 (ps/km/nm) withregard to a wavelength of the input pulsed light. Polarizationmaintaining optical fibers are preferable, although non-polarizationmaintaining type is usable. The broadband optical spectrum-generatingoptical fibers 24 used in this embodiment are polarization maintaininghighly nonlinear normal dispersion shifted fibers having an adjustedlength of 3 m and a mode field diameter of 3.5 μm, a nonlinearcoefficient of 15 (W⁻¹km⁻¹)m and a magnitude of wavelength dispersionequal to −12.7 (ps/km/nm) with regard to a wavelength 1.55 μm of theinput pulsed light. For example, in the case of silica glass opticalfibers, the nonlinearity is enhanced with the smaller cross sectionalarea of light propagation and the greater quantity of germanium dioxide(GeO₂) added to the core. The nonlinearity is thus adjusted byregulating the cross sectional area of light propagation and thequantity of the additive in the case of the polarization maintaininghighly nonlinear dispersion shifted fibers. The broadband opticalspectrum-generating optical fibers 24 of the embodiment may be producedby a typical manufacturing method of optical fibers.

The lens 26 used in this embodiment has a diameter of 2 (mm) and a focaldistance of 2 (mm) and enables the pulsed light from the ultrashortpulse fiber laser 22 to enter the broad optical spectrum-generatingoptical fibers 24. The output end of the pulsed light from theultrashort pulse fiber laser 22 is the optical fibers. The lens 26 maybe omitted when the output end of the ultrashort pulse fiber laser 22 isdirectly connected to the broadband optical spectrum-generating opticalfibers 24.

The broadband optical spectrum generating apparatus 20 of the embodimentthus constructed has the following features. FIGS. 2 through 4 aregraphs showing observed output spectra from the broadband opticalspectrum-generating optical fibers 24, when pulsed light having a meanoutput of 30 (mW), a pulse width of 110 (fs), and an energy level of 625(pJ) per pulse is input from the ultrashort pulse fiber laser 22 intothe broadband optical spectrum-generating optical fibers 24 of diversefiber lengths. As shown in the graphs of FIGS. 2 through 4 with regardto the fiber length of 1 (m) to 5 (m), super continuum widely broadenedin a wavelength band of 1400 (nm) to 1750 (nm). No experiment wasspecifically performed with regard to optical fibers having a fiberlength of greater than 5 (m). Such experiments are, however, notrequired, since it is empirically known that the greater fiber lengthgives the more favorable super continuum. The optical spectrum tends tobe more widely broadened with the narrower pulse width, the greater peakintensity, the higher nonlinearity, and the smaller magnitude ofwavelength dispersion. When the pulse width and the magnitude ofwavelength dispersion are fixed, the optical spectrum is determined bythe product of the peak intensity and the nonlinear coefficient. FIG. 5is a graph showing variations in wavelength of the observed outputspectra from the broadband optical spectrum-generating optical fibers 24plotted against the output of the ultra short pulse fiber laser 22. Acurve of solid line with open circles represents a variation in lowerlimit wavelength of the observed spectra, and a curve of one-dot chainline with open squares represents a variation in upper limit wavelengthof the observed spectra. Both plots should be read on the coordinateaxis on the left side. A curve of broken line with closed circlesrepresents a variation in difference between the lower limit wavelengthand the upper limit wavelength of the observed spectra. This plot shouldbe read on the coordinate axis on the right side. As shown in the graph,practical super continuum is obtained when the ultrashort pulse fiberlaser 22 has an output of not less than 10 (mW).

FIGS. 6 and 7 are spectrograms showing the relationship between thewavelength and the time of appearance of the observed output spectrumfrom the broadband optical spectrum-generating optical fibers 24 havingfiber lengths of 1 m and 5 m under the same conditions as those of FIGS.2 through 4. FIG. 8 is a spectrogram showing the relationship betweenthe wavelength and the time of appearance of the observed outputspectrum from optical fibers of a comparative example, which were usedin place of the broadband optical spectrum-generating optical fibers 24,under the same conditions as those of FIGS. 2 through 4. The opticalfibers of the comparative example used here are polarization maintaininghighly nonlinear dispersion shifted fibers having an adjusted length of5 (m) and a mode field diameter of 3.5 (μm), a nonlinear coefficient of21 (W⁻¹km⁻¹), and a magnitude of wavelength dispersion equal to a value1 (ps/km/nm) with regard to a wavelength 1.55 μm of the input pulsedlight. As shown in the graph of FIG. 8, spectra of significantlydifferent wavelengths appear simultaneously in the comparative example.In the graphs of the embodiment, on the other hand, there are linearrelationships between the wavelength and the time of appearance of thespectra. The linearity is affected by the magnitude of wavelengthdispersion and the length of the broadband optical spectrum-generatingoptical fibers 24. The results of the experiment show that the favorablemagnitude of wavelength dispersion is about −12.7 (ps/km/nm) and thefavorable length is about 5 (m).

The broadband optical spectrum generating apparatus 20 of the embodimentdiscussed above uses the ultrashort pulse fiber laser 22 that generatesthe pulsed light having the pulse width in the unit of picosecond tofemtosecond, and the broadband optical spectrum generating-opticalfibers 24 that are characterized by the nonlinear coefficient of notless than 10 (W⁻¹km⁻¹) and the magnitude of wavelength dispersion of notgreater than the value 0 (ps/km/nm) with regard to the wavelength of theinput pulsed light. This construction accordingly generates the supercontinuum widely broadened to the wavelength band of 1400 nm to 1750 nmuseful for optical communication. The generated broadband opticalspectrum is chirped to the linear characteristic and has the linearrelationship between the wavelength and the time of appearance of thespectrum. This feature desirably simplifies subsequent processingincluding a series of processing in the course of extracting an opticalspectrum of an arbitrary wavelength.

The broadband optical spectrum generating apparatus 20 of the embodimentuses the single ultrashort pulse fiber laser 22, thus attaining sizereduction and simplified construction of the apparatus. In the broadbandoptical spectrum generating apparatus 20 of the embodiment, the lengthof the broadband optical spectrum-generating optical fibers 24 rangesfrom 1 to 10 m and is preferably about 5 m. Compared with the prior artapparatus using the optical fibers of several hundred meters or evenseveral kilometers, this arrangement attains the effective sizereduction.

The following describes a broadband optical spectrum generatingapparatus 20B in a second embodiment of the present invention. FIG. 9schematically illustrates the construction of the broadband opticalspectrum generating apparatus 20B of the second embodiment. Thebroadband optical spectrum generating apparatus 20B of the secondembodiment has pulse compressing optical fibers 28 for compressing thetime width of the pulsed light output from the ultrashort pulse fiberlaser 22, which is located before the broadband opticalspectrum-generating optical fibers 24, in addition to the structure ofthe broadband optical spectrum generating apparatus 20 of the firstembodiment. The other constituents of the broadband optical spectrumgenerating apparatus 20B of the second embodiment, that is, theultrashort pulse fiber laser 22, the broadband opticalspectrum-generating optical fibers 24, and the lens 26, are described inthe first embodiment and thus not specifically discussed here again.

Optical fibers having abnormal dispersion characteristics are preferablyapplied for the pulse compressing optical fibers 28 included in thebroadband optical spectrum generating apparatus 20B of the secondembodiment. The preferable characteristics include a nonlinearcoefficient in a range of 2 to 10 (W⁻¹km⁻¹) and a magnitude ofwavelength dispersion of not less than a value 4 (ps/km/nm) with regardto a wavelength of the input pulsed light, and a length of 10 to 50(cm). The optical fibers used in the second embodiment are characterizedby a nonlinear coefficient of 4.3 (W⁻¹km⁻¹), a magnitude of wavelengthdispersion equal to a value 12 (ps/km/nm), and a length of 16 (cm).

The broadband optical spectrum generating apparatus 20B of the secondembodiment thus constructed has the following characteristics. FIG. 10is a graph showing the width of the output pulse plotted against thelength of the pulse compressing optical fibers 28 in one experiment. Inthe experiment, a pulse having a pulse width of 100 (fs) was output fromthe ultrashort pulse fiber laser 22 with a laser output of 60 (mW) and afiber coupling light intensity of 38 (mW). The length of the pulsecompressing optical fibers 28 was varied in a range of 10 to 50 (cm). Asshown in the graph, the pulse width was not greater than 50 (fs) againstany fiber length. The minimum pulse width was observed in the case ofthe pulse compressing optical fibers 28 having a length of 15 to 17(cm). FIG. 11 shows super continuum output from the broadband opticalspectrum-generating optical fibers 24 included in the broadband opticalspectrum generating apparatus 20B of the second embodiment, when thepulse compressing optical fibers 28 used have a length of 16 (cm). Supercontinuum output from the broadband optical spectrum-generating opticalfibers 24 included in the broadband optical spectrum generatingapparatus 20 of the first embodiment is also shown in the graph of FIG.11, for the purpose of comparison. As shown in the graph, the broadbandoptical spectrum generating apparatus 20B of the second embodimentgenerates the super continuum widely broadened in a frequency band of1300 (nm) to 1850 (nm). The length of the pulse compressing opticalfibers 28 tends to be shortened with the higher peak intensity of theinput pulsed light and elongated with the wider pulse width.

In the broadband optical spectrum generating apparatus 20B of the secondembodiment discussed above, the pulse compressing optical fibers 28 forcompressing the time width of the pulsed light output from theultrashort pulse fiber laser 22 is provided before the broadband opticalspectrum-generating optical fibers 24. Compared with the structurewithout the pulse compressing optical fibers 28, this structuregenerates the more widely broadened super continuum. The structure ofthe broadband optical spectrum generating apparatus 20B of the secondembodiment is identical with that of the broadband optical spectrumgenerating apparatus 20 of the first embodiment, except the pulsecompressing optical fibers 28 provided before the broadband opticalspectrum-generating optical fibers 24. Like the broadband opticalspectrum generating apparatus 20 of the first embodiment, the secondembodiment generates the broadband optical spectrum chirped to thelinear characteristic and thereby exerts the effect of facilitating thesubsequent processing including the series of processing in the courseof extracting an optical spectrum of an arbitrary wavelength. The secondembodiment uses the single ultrashort pulse fiber laser 22 and thusexerts the effects of size reduction and simplified construction of theapparatus.

The following describes a pulsed light generating apparatus 30 includingthe broadband optical spectrum generating apparatus 20 of the firstembodiment or the broadband optical spectrum generating apparatus 20B ofthe second embodiment. FIG. 12 schematically illustrates theconstruction of the pulsed light generating apparatus 30 in oneembodiment of the present invention. The pulsed light generatingapparatus 30 of the embodiment includes a wavelength tunable filter 32,which is connected to the output of the broadband optical spectrumgenerating apparatus 20 of the first embodiment or the broadband opticalspectrum generating apparatus 20B of the second embodiment and varies asetting of transmission wavelength in response to a driving signaloutput from a computer 34.

The wavelength tunable filter 32 may utilize, for example, a diffractiongrating or a prism as in the case of a spectroscope or an acousto-opticlight modulator. The wavelength tunable filter 32 of this embodimentutilizes the acousto-optic light modulator. The acousto-optic lightmodulator supplies a driving signal to an internal driver, so as toreadily vary the transmission wavelength. In the structure of theembodiment, the transmission wavelength set in the wavelength tunablefilter 32 is freely adjustable in response to a control signal outputfrom the computer 34 and given to the internal driver of theacousto-optic light modulator.

In the pulsed light generating apparatus 30 thus constructed, thetransmission wavelength set in the wavelength tunable filter 32 isregulated with regard to the widely broadened super continuum of 1400(nm) to 1750 (nm) generated by the broadband optical spectrum generatingapparatus 20 of the first embodiment or the widely broadened supercontinuum of 1300 (nm) to 1850 (nm) generated by the broadband opticalspectrum generating apparatus 20B of the second embodiment. Thistechnique ensures output of the pulsed light having a desired wavelengthin the band of the resulting super continuum. The computer 34 functionsto regulate the transmission wavelength set in the wavelength tunablefilter 32. Any desired technique is thus applicable to output the pulsedlight of the desired wavelength. An exemplified technique automaticallyand successively varies the transmission wavelength from the short tothe long or from the long to the short in the band of the supercontinuum.

In the pulsed light generating apparatus 30 of the embodiment, thecomputer 34 functions to regulate the transmission wavelength set in thewavelength tunable filter 32. One modified application may omit thecomputer 34 from the pulsed light generation apparatus 30.

The above embodiment is to be considered in all aspects as illustrativeand not restrictive. There may be many modifications, change, andalterations without departing from the scope or sprit of the maincharacteristics of the present invention. All changes within the meaningand range of equivalency of the claims are therefore intended to beembraced therein.

1. A pulsed light generating apparatus that generates pulsed light of adesired wavelength out of a predetermined wavelength band, saidapparatus comprising: a pulsed light source that outputs pulsed lighthaving a pulsed width in a unit of picosecond to femtosecond; spectrumgenerating optical fibers that have characteristics of normaldispersion, that is, a nonlinear coefficient of not less than 10(W⁻¹km⁻³) and a magnitude of wavelength dispersion of not greater than avalue 0 (ps/km/nm) with regard to a wavelength of the pulsed lightgenerated by said pulsed light source, are adjusted to have a length ina range of 1 (m) to 10 (m), and convert the pulsed light input from saidpulsed light source into a broadband optical spectrum; a wavelengthtunable filter that allows transmission of an optical spectrum of anarbitrary wavelength out of the broadband optical spectrum output fromsaid spectrum generating optical fibers; and a wavelength selector thatselects a wavelength of the optical spectrum transmitting through saidwavelength tunable filter, in response to an electric signal.
 2. Apulsed light generating apparatus in accordance with claim 1, whereinsaid wavelength tunable filter is an acousto-optic wavelength tunablefilter.
 3. A pulsed light generating apparatus in accordance with claim1, wherein said pulsed light source outputs pulsed light having acentral wavelength of about 1556 (nm), a pulse width in a range of 10 to1000 femtoseconds, and a peak intensity of not less than 1 (kW).
 4. Apulsed light generating apparatus in accordance with claim 1, whereinsaid spectrum generating optical fibers are characterized by a nonlinearcoefficient around 15 (W⁻¹km⁻¹) and a magnitude of wavelength dispersionaround −10 (ps/km/nm) and have a length of approximately 5 (m).
 5. Apulsed light generating apparatus in accordance with claim 1, saidapparatus further comprising: pulse compressing optical fibers that havecharacteristics of abnormal dispersion with regard to the wavelength ofthe pulsed light generated by said pulsed light source, and compress atime width of the pulsed light output from said pulsed light source tooutput the time width-compressed pulsed light to said spectrumgenerating optical fibers.
 6. A pulsed light generating apparatus inaccordance with claim 5, wherein said pulse compressing optical fibersare characterized by a nonlinear coefficient in a range of 2 to 10(W⁻¹km⁻³) and by a magnitude of wavelength dispersion of not less than avalue 4 (ps/km/nm) and have a length in a range of 10 to 50 (cm).